1. Field of the Invention
The present invention relates generally to a hydroball in-core instrumentation system and, more specifically, to a stepping apparatus for regulating feed of a hydroball string to a gamma counter.
2. Description of the Related Art
A hydroball in-core instrumentation system and method of operation are described in U.S. patent application Ser. No. 024183, filed April 24, 1987, now abandoned, which is incorporated herein by reference. The aforementioned system includes detector string transfer tubes, leading to and from the reactor vessel, provided for each detector string. Typically, there are approximately fifty such tubes and forty-eight detector strings for a typical two or three loop plant.
The structure pointed out in the above-mentioned patent application uses detector strings which are capable of being passed through very small diameter tubes with short bend radii without inducing excessive drag and pressure drop. The tubes pass through a heat exchanger or flow cooler tank. Tubes passing through the heat exchanger are connected to a system of solenoid valves.
A detector string storage position is provided between a transfer device and the solenoid valves. The detector string storage position may be simply that section of the tubes extending between the valves and the transfer device. Each detector string may be individually and selectively transported into or removed from its original storage position by means of the transfer device. A fluid handling system generates the flow of primary coolant for selectively moving the detector strings for insertion into and withdraw from the vessel and for the handling or transport functions associated with the in-core instrumentation system readout room.
A new detector string is inserted through the detector loading funnel after opening the valve between the funnel and the transfer device. The transfer device is set to a position which allows entry of the new detector string, while appropriate valves are open to permit the detector circulation pump to produce a flow which impels the detector string into a counter, whereupon a forward end of the detector string comes to rest against a stop. The transfer devices is then moved to its connection position corresponding to the position of the new detector string to pass into the detector storage position.
Each string used in the hydroball instrumentation system uses stainless steel balls on a twelve foot cable which move in and out of the reactor core under the force of the reactor coolant flow. The forty-eight strings in the system are to be counted by one gamma counter. A transfer device is necessary to align each of the fifty tubes containing the strings with the one tube of the gamma counter. The device must be able to operate at about 2,250 psi (15.5 MPa).
After irradiation in the reactor core, each ball on every string must be passed by the counter to register activity. The balls must be accurately positioned for a known and repeatable time interval at the counter. The string is inside a high pressure boundary in a radioactive system, and thus, the pressure boundary must be maintained.